The invention relates generally to heating apparatus and in particular to wood or coal burning heating apparatus having a high heat conversion efficiency.
Wood-burning stoves have been available for centuries. Perhaps the most well-known early wood-burning stove is the Franklin stove which, while being practical for its time, burned wood inefficiently. That stove, like most cast iron stoves available prior to about 1976, provides for updraft combustion, such as is found in a fireplace and in which the volatile gases (volatiles), which are driven off as the wood burns, are generally left unburned. The unburned volatiles remain for two reasons, first because the gases, by the time they have left the wood, are generally too cool for secondary combustion and second, because oxygen that is admitted to the stove or fireplace is usually consumed by the coals at the base of the fire mass, causing the gases to rise through an oxygen-deficient atmosphere. The loss of the hot, unburned volatile gases is a serious problem, becuase they represent approximately half of the total heat value of the wood. It is as though one were to run an open line of natural gas up a chimney without bothering to ignite it first. In addition, the volatile gases given off from the wood without being ignited may condense on the cool sides of long metal flue pipes and drip out as creosote which may sometimes be inadvertently and dangerously burned, in their solid creosote form, as a chimney fire.
One effective method and apparatus for reducing the volatiles is found in a horizontal combustion heating apparatus wherein the flames move horizontally in the primary combustion zone. This is radically different from typical updraft combustion and is the basis of the more efficient operation of a stove manufactured by Vermont Castings, Inc., the assignee of this invention, a stove which has been sold since at least the first quarter of 1976 under the trade name "DEFIANT". The DEFIANT parlor stove aids the burning of the volatile gases in several ways. First, by using horizontal combustion, the gases are forced to pass close to the hot coals which maintain sufficiently high temperatures to ignite them. In addition, a manually controlled secondary air source, which is segregated from the primary air source, provides air which is ducted down a tube integral with the heated fireback of the primary combustion chamber, which is heated by the hottest part of the fire on two sides, and this channel preheats the air to maintain the air at the elevated temperatures required for combustion. Thus, oxygen from the secondary source is led into the secondary combustion chamber through numerous air ports to mix with the combustible gases and to provide secondary combustion. In the "DEFIANT", the secondary source of air is provided in the secondary combustion chamber along an outside wall of the chamber, away from the primary combustion chamber.
In addition, behind the fireback which is provided at the back of the primary combustion chamber, a circuitous path is provided by smoke baffles or guide plates. The circuitous path generally has a plurality or smoke passages, which conduct the smoke through the passages, back and forth along the back of the apparatus and upwardly toward the exit at the flue collar. Since the heat of the flue gases is considerable, significant heat transfer occurs from the flue gases to the outside surfaces of the stove, which in turn is given off into the room rather than being lost up the chimney. In addition, the circuitous path aids in maintaining a higher temperature in the primary combustion chamber which in turn aids in burning the volatile gases driven off from the wood. Thus, in the "DEFIANT", a large heat output is available. Further, in the "DEFIANT", the fire is controlled by not only the structure of the apparatus but by a thermostatically controlled input port through which the primary air is supplied.
In the "DEFIANT", as in many cast stoves, some of the components defining and enclosing the primary combustion chamber are subjected to significant high temperatures. In particular, in the "DEFIANT", the large cast iron fireback is, at its lower portion, exposed to the primary fire (the combustion region); however, the upper portion of the fireback, being spaced further away from the fire, is not as hot. The resulting different thermal expansions subject the fireback to large, thermally induced, mechanical stresses.
In addition, other components of the stove are subject to mechanical and thermal stress due to the high temperatures found in the stove and hence, in time, may fatigue or otherwise wear. In the "DEFIANT", as in other cast iron heating apparatus, the construction is one whereby, once the apparatus is completely secured in an assembled condition, the internal component parts are not removable from the stove without completely disassembling the apparatus, including its outside frame. Thus, if one interior part fails, for whatever reason, the stove must ordinarily be completely disassembled to replace the failed mechanical part. This is typically a mechanical operation which should be performed only at an adequately equipped service facility, for example, the original manufacturing facility. This procedure is both expensive for the manufacturer if he warrants the product or for the customer if the product is not warranted. In addition, there may be a significant period of time during which the consumer is without the stove. If the failure occurs during the winter months, it would likely pose a significant problem for the consumer.
It is therefore a principal object of the invention to provide a heating apparatus in which thermally induced material stresses are significantly reduced and in which interior components can be replaced simply and easily. Other objects of the invention are to significantly reduce any tendency for mechanical failure of the fireback due to thermally induced stresses, to provide an interior component construction for the heating apparatus wherein the interior components can be removed by the consumer, and to provide a heating apparatus which is reliable and which has a long expected operating life.